MODIS-derived Snow Metrics

Summary

The National Park Service and Geographic Information Network of Alaska (GINA) developed an algorithm to derive snow cover climatology for Alaska using the MODIS snow cover daily product.

The algorithm is two-fold and involves both data processing and the derivation of snow cover metrics. Terra MODIS snow cover daily 500m grid data version 5 (MOD10A1.v005) are processed to reduce cloud obscuration through iterations of cloud reduction methods that include spatial, temporal, and snow cycle filtering. A total of 12 metrics (e.g. date of first snow, date of persistent snow cover) for each pixel are calculated.

Data Sources

The MODIS Terra Snow Cover Daily L3 Global 500m Grid data (MOD10A1.v005) from the National Snow and Ice Data center (NSIDC) is used to calculate the snow metrics. Data description can be accessed at NSIDC Data Portal.

The MOD10A1.v005 data contains snow cover, snow albedo, fractional snow cover, and Quality Assessment (QA) data along with corresponding metadata. It consists of 1200 km by 1200 km tiles of 500 m resolution data gridded in a sinusoidal map projection. For our purposes, we downloaded 24 tile files covering the Alaska region, created a mosaic, reprojected them into the Alaska Albers Projection (NAD83), and output the four scientific fields of snow cover, snow fraction, snow quality, and snow albedo into four single band GeoTIFF files, respectively.

MOD10A1.v005 data cover the timeframe from 2000.02.24 to 2017.01.01. NSIDC released the MODIS Terra Snow Cover Daily L3 Global 500m Grid data version 6 (MOD10A1.v006) recently. The new data covers from 2000.02.24 to present. GINA and NPS have developed the MODIS_Snow_metrics_algorithm version 006 to calculate the snow metrics against MOD10A.v006. New snow metrics based on version 6 MOD10A1 will be available soon.

The snow metrics are also available through GINA's http server. Users should be aware of that files provided by the http server are in ENVI format. Each snow year metrics is composed of one image file and one header file.

Snow-metrics data file defines the following 12 snow metrics:

first_snow_day, first day of the full snow season (FSS start day)

last_snow_day, last day of the full snow season (FSS end day)

fss_range, last_snow_day-first_snow_day +1

longest_css_first_day, first day of the longest CSS segment (CSS start day)

longest_css_last_day, last day of the longest CSS segment (CSS end day)

Days reported as metrics are counted as day of year beginning on Jan.1 of the year that precedes the name of the snow year. For example, the days reported in the snow metrics of the 2010 snow year are counted from Jan.1, 2009.

The values and their descriptions in mflag are defined in Table 1. Definition of mflag